This invention deal with the combination of a linear bearing and a guide rail therefor, for use in moving an object along a rectilinear path with a minimum of friction. More specifically the invention is directed to such linear bearing apparatus of the type incorporating groups of recirculating antifriction balls and wherein the direction of action of the load transmitted can be either downward, upward, or opposite lateral directions. The linear bearing apparatus according to the invention finds applications in machine tools, machining centers, and a variety of other pieces of machinery or equipment wherein one part is to be moved linearly relative to another.
Japanese Patent Laying-Open No. 55-72912, filed by the assignee of the instant application, discloses linear bearing apparatus of the type in question. It comprises a bearing body of inverted-U-shaped cross section mounted astride a guide rail via groups of recirculating antifriction balls for rolling movement thereon. Although this prior art apparatus can largely well accomplish the purposes for which it is intended, it has proved to have certain problems in connection with the guide rail and some other components.
In the noted prior art apparatus the guide rail takes the form of a heavy beam of approximately square section. The guide rail of this construction does not necessarily lend itself to combined used as a structural member, such as a beam or girder, of a building or as a framing member of a machine or equipment. Discussed hereinbelow are the reasons for this.
As the load on the linear bearing acts thereon in various possible directions, the forces are transmitted directly to the guide rail supporting the bearing. The guide rail must therefore be of sufficient rigidity to remain undeformed by such stresses. For use as a structural or framing member, moreover, the guide rail must also withstand bending and twisting forces. The known guide rail meets those requirements to some extent solely by virtue of its great cross sectional size. This conventional solution makes it impossible to use the guide rail as a beam or girder mounted on discontinuous supports, for the guide rail would sag between the supports under its own weight. The deformation of the guide rail is a serious impediment to the proper rolling of the linear bearing thereon. Thus the known linear bearing apparatus has been limited as to its places of installation.
Another problem concerns a pair of end covers fastened to the opposite ends of the bearing body to provide parts of the closed paths for the recirculation of the antifriction balls. The end covers in the prior art apparatus have been each integrally molded of plastics material. The one-piece molding of each end cover is objectionable because of its complex shape. Having approximately the cross sectional shape of the bearing body, the end covers have formed therein not only curved grooves serving as parts of the closed ball paths but also annular depressions and other recesses. Much difficulties have been involved in the production of molds for such complex shapes and in the control of the heat and pressure conditions for the molding of the end covers therein. Some errors in the dimensions of the molded end covers have therefore been almost unavoidable, resulting in the hindrance of the smooth rolling of the balls through the curved grooves in the end covers. The manufacture of such unitary end covers has also been very time-consuming and so added considerably to the cost of the complete bearing.
A further problem with the prior art linear bearing apparatus relates to a unitary cage rollably holding the antifriction balls on raceways formed on the inside surfaces of the bearing body. Generally horseshoe-shaped in cross section to fit the interior contours of the bearing body, the cages has slots formed longitudinally therein to allow the balls to make rolling engagement with raceways on the guide rail. The cage is fabricated from sheet steel of means of a press. By reason of the complex shape of the cage, however, it has been difficult to create the slots in exact positions thereon. Difficulties have also been encountered in withdrawing the punches from the slots as they are formed in sloping parts of the cage.
The unitary construction of the conventional cage has also tended to give rise to its dimensional errors and mounting errors. Such errors make it impossible to rigidly hold the cage relative to the bearing body and other pertinent parts of the bearing. Thus the manufacture of linear bearings of any great longitudinal dimension has been virtuelly inhibited. Further the unitary cage with the dimensional or mounting errors has been easy to deflect or become displaced under load, impeding the proper rolling of the antifriction balls and, in consequence, the proper movement of the bearing along the guide rail with a minimal rolling friction.